In one form of such micropump, three or more oscillators are disposed along a fluid channel space extending from an input port to an output port and are driven for oscillation so that they are sequentially displaced in phase. A first oscillator is located adjacent to the input port while a second oscillator is located adjacent to the output port, and a third set of oscillators including one or more oscillators are disposed between the first and the second oscillator for reducing/enlarging the fluid channel space. When the second oscillator closes the output port and the first oscillator is driven to open the input port, the third set of oscillators are driven for suction. When the second oscillator is driven in a direction to open the output port while the first oscillator is driven in a direction to close the input port, the third set oscillators are driven for discharge. Subsequently, the described sequence is repeated. By driving the first, the third set of and the second oscillators in a sequential order with a given phase difference therebetween, a fluid can be driven from the input to the output port. One of such micropumps is disclosed in Japanese Laid-Open Patent Application No. 149,778/1990.
In the micropump disclosed in this Laid-Open Application, the first oscillator oscillates in a manner to open or close the connection between the input port and the fluid channel space. However, the drive applied to the oscillator in a direction to close the connection therebetween tends to be low, and whenever a high pressure is applied to the input port, such pressure is effective to force the oscillator open, resulting in a failure to close the channel space and causing a propagation of the high pressure at the input port to the output port. For example, when an input pressure at the input port is subject to a fluctuation, the outcome is that a high pressure appears at the output port for an interval corresponding to the high level of the input pressure, resulting in a fluctuation in the output delivered from the output port. For most applications, it is necessary that the micropump maintains a constant flow rate (or a constant velocity of flow) though the flow rate (the amount of flow per unit time or velocity of flow) is very low. Thus, it is desirable that the constant velocity of flow be maintained despite any fluctuation in the pressure appearing at the input port. By way of example, when a reagent is continuously supplied at a given rate for purpose of a continuous chemical reaction or analysis, when the supply is controlled in terms of a pumping time in order to meter a small quantity, or when a small quantity of liquid medicine is to be administered, by injection, to a patient, a close control over the flow rate being supplied is required. Obviously it is desirable that a micropump be compact, easily assembled, and has reduced variation from product to product.